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Summary: Frequently Asked Questions.
Archive-name: perl-faq/ptk-faq/part4
Posting-Frequency: monthly
Last-modified: Date: Sat May 31 16:48:37 1997
Version: 1.00_07

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Perl/Tk FAQ part 4 of 5 - More Perl/Tk     

 13. What are some of the primary differences between Tcl/Tk and Perl/Tk? 
 Considering that both interpreters/(compiler) for Tcl and Perl were written in
 C for original use on Unix computers it is not surprising that there are some
 similarities between the two languages. 
 Nevertheless, there are a large number of differences between the Tcl
 language and the Perl language. One thing to keep in mind is that to build,
 install, and use Perl/Tk one does not need to have Tcl/Tk on hand at all.
 Perl/Tk is completely independent of Tcl/Tk. 
 Tom Christiansen (a definite perl proponent) has put up a web page that
 elucidates some critical technical differences between Tcl and Perl at:
 Within each language there is Tk - a widget Toolkit. One must be careful that
 some of the Tcl/Tk widget names and options have been modified slightly in
 the perl/Tk language. 
 With Tk-b9.01 (and higher) a great many functions (method calls actually)
 start with an upper case letter and continue with all lower case letters (e.g.
 there is a perl/Tk Entry widget but no entry widget), and many
 configuration options are all lower case (e.g. there is a perl/Tk 
 highlightthickness option but no highlightThickness option). Thus
 if you are having trouble converting a script check your typing. (there is a
 script b9names to help). There is also a tcl2perl script (discussed later). 
 The html docs that get created during the build of perl/Tk ought to help clarify
 most any language difference. While the following table does not cover all the
 differences it is hoped that it will prove useful, especially to those people
 coming from a primarily Tcl/Tk programming background. These are some of
 the common Tcl->Perl stumbling points: 
 what              Tcl/Tk                 Perl/Tk
 variable          set a 123              $a = 123; or $a = '123';
 re-assignment     set b $a               $b = $a;
 lists/arrays      set a {1 2 fred 7.8}   @a = (1,2,'fred',7.8);
 re-assignment     list set b $a          @b = @a;
 associative       set a(Jan) 456.02      %a = ('Jan',456.02,'Feb',534.96);
  arrays           set a(Feb) 534.96
 re-assignment     foreach i \            %b = %a;
                    [array names a] {
                    set b($i) = $a($i) }
 Note on the above examples:
 In Tcl the scalar, list, and array variable 'a' will overwrite each 
 previous assignment.
 In Perl $a, @a, %a are all distinct (occupy separate namespaces).
 expressions       set a [expr $b+$c]     $a = $b+$c;
 increment         incr i                 $i++; or ++$i;
 declare           proc plus {a b} {      sub plus { my($a,$b) = @_;
  subroutines       expr $a + $b }         $a+$b; }
 variable scope    local default          global default
                   override w/ "global"   override w/ "my" (or "local")
 call              plus 1 2               &plus(1,2); #or
  subroutines                             plus(1,2);  #OK after sub plus
 statement sep     newline or at ";"      ";" required
 statement         "\" - newline          none required
 verbatim strings  {}                     ''
  e.g.             {a \ lot@ of $stuff}   'a \ lot@ of $stuff'
 escaped strings   ""                     ""
  e.g.             "Who\nWhat\nIdunno"    "Who\nWhat\nIdunno"
 STDOUT            puts "Hello World!"    print "Hello World!\n"
                   puts stdout "Hello!"   print STDOUT "Hello!\n"
 Note also that Tcl/Tk has a built-in abbreviation completion mechanism that
 lets you specify short hand, e.g. 
    canvas .frame.canvas -yscrollcommand ".frame.scroll set" ; #Tcl/Tk OK
    canvas .frame.canvas -yscroll ".frame.scroll set" ;        #Tcl/Tk also OK
    $canvas=$main->Canvas(-yscroll => ['set',$scroll]);  #ERROR perl/Tk
    $canvas=$main->Canvas(-yscrollcommand => ['set',$scroll]); #perl/Tk OK
 You may get around this with the perl package in certain
 circumstances. For example: 
    require '';
    %foo = ();
    $canvas=$main->Canvas($foo{'-yscroll'} => ['set',$scroll]); #perl/Tk OK
 In Perl you can emulate the Tcl unknown proc (through the perl AUTOLOAD
 mechanism) as follows: 
     use Shell;
     print($p = man(-k => bitmap));
 Which is equivalent to what you would get if you typed: 
     man -k bitmap
 >From within tclsh or wish. (Thanks to Ilya Zakharevich 
 <> for pointing out this feature. ;-) 
 14. How do I install new scripts | modules | extensions? 
 (Thanks to Ilya Zakharevich <> for pointing out
 that perl code comes in a variety of flavors and some code requires more work
 than others to install. Hence I have expanded this topic and will refer to three
 distinct categories here: Scripts Modules and Extensions:) 
 A "self-contained" script needs little modification (in principle!) to run. It is a
 good idea to check the #! line at the very top of the file to reflect your local
 perl setup (e.g. #!/usr/bin/perl -w (change to) 
 #!/usr/gnu/local/perl -w or what have you). There are allegedly "more
 portable" ways to invoke the perl interpretor as well - they are more fully
 documented in the perl FAQ and the perlrun(1) man page, however. 
 Other things you do not want to forget when trying to run a perl script include
 giving yourself permission to do so, e.g.: 
     chmod u+x newscriptname
 You also want to be sure your DISPLAY environment variable is set up
 properly when attempting to run a perl/Tk script. You may also need to look at
 the xhost(1) or the xauth(1) man pages for setting up your X-display
 If you are still experiencing difficulty check to be sure that extraneous
 /newsgroup|e-mail|HTML headers|footers|markup//; are not in the file and
 that you have on hand all that is requireed or useed by the script (if not you
 may need to install a module - or even a perl4 style file). 
 Check out the module - make sure it is OK and will run on your system - does
 it require a specific location? For testing purposes (always a good idea) or if
 you do not have root priveleges set the file in a directory that you do have
 write access to and try to include it in a test script. Assuming you have a
 module to test called "" and are simply running the test script in the
 same directory as the module begin by adding to the @INC array like so: 
     #!/usr/bin/perl -w
      BEGIN { @INC = ("$ENV{'PWD'}",@INC); }
      use Tk;
      use Foo;
     #!/usr/bin/perl -w
      use lib $ENV{PWD};
      use Tk;
      use Foo;
 Another approach is to set either the PERLLIB or PERL5LIB environment
 variable from your shell. This method allows invoking the test script from
 within a number of different directories without having to edit a hard coded 
 use lib or push(@INC,".") kind of statement within the script. Yet
 another way to do it is with the -I switch on the command line like so: 
     perl -Ipath/to/Foo -e fooscriptname
 After a successful test; if you are a system administrator, or have root
 priveleges, or are modifying your own copy of perl; then copy it to the 
 perl5/Tk directory. Depending on how the module was written it should be
 possible to use it either with the use Tk; statement itself or with an explicit 
 use Tk::Foo; (for module perl5/Tk/ 
 Extensions (Overgrown Modules)
 These may come as a multi-file kit (tape archive usually) and may require a C
 compiler for part of the installation (perl/Tk itself falls into this category).
 You know you have an Overgrown Module (Extension) when there is one or
 more files with an .xs extension (perl->C meta code) and a Makefile.PL
 (perl->make meta code). One invokes the perl MakeMaker on the file called 
 Makefile.PL in order to create a Makefile via: 
     perl Makefile.PL
 You may now run make on the resultant Makefile - but the details of this
 process are module dependent and should be documented in a README or an 
 INSTALL file. A very standard perl extension requires 4 (or 5 if making static)
 standard commands to make and install: 
     perl Makefile.PL
     make test
     make install
 If you have the appropriate CPAN and FTP modules already installed you can
 retrieve a module from CPAN and carry out all of the above steps with a perl
 one-liner like this: 
     perl -MCPAN -e 'install "Foo"'
 15. How do I write new modules? 
 You might want to start by poking around your perl/Tk build directory. Is there
 something there that already does what you want? Is there something that is
 reasonably close - but only requires minor modification? 
 Next go through the various perl documents - including the FAQ as well as
 the various relevant man pages: perlmod(1), perlobj(1), perlbot(1),
 (and please don't forget: perlpod(1)!) 
 Post your idea to and discuss it with others - there might
 very well be someone working on an approach already. A clear explanation of
 all the stuff that gets put into a module was posted to the mailing list and can
 be found in the archive at:
 Also, be sure to check out a recent version of the official Module List that Tim
 Bunce <> and Andreas Koenig 
 <a.koenig@franz.ww.TU-Berlin.DE> maintain and post to 
 comp.lang.perl.announce periodically. The list is also available at any CPAN
 ftp site as well as:
 Finally ready to ship? Small (perl/Tk) modules have been posted directly to Big modules may require ftp distribution (see upload info at
 one of the CPAN sites) then make your announcement to
 and possibly to comp.lang.perl.announce. 
 16. Composite Widgets. 
 Composite widgets combine the functions of two or more widget primitives
 into something that is not quite a stand alone program but is something that
 may prove very useful for inclusion in your own scripts. A variety of
 composite widgets have been written and many are still being worked on.
 Many come bundled with your perl/Tk distribution kit, and some are simply
 posted to It is quite common to have composite widgets
 written in perl modules - usually in terms of the Tk widget primitives.
 Graphical examples of some of the composites discussed here can be seen by
 GUI browsers at:
 16.1. How do I get a Dialog box? 
 For things like a simple "are you sure?" dialog box you might want to take a
 look at perl5/Tk/ This module may be invoked with require
 Tk::Dialog; etc. - there are much more extensive directions inside the
 comment fields at the top of the file itself. The module has a lot
 of options and has a tutorial driver script in perl5/Tk/demos/dialog. is also used by the perl5/Tk/demos/widget demo. In particular
 look at perl5/Tk/demos/widget_lib/ and for
 examples of how one makes use of Tk::Dialog. A snippet of a script that uses
 this module could look like: 
     require Tk::Dialog;
     my $mw = MainWindow->new;
     my $D = $mw->Dialog(
                  -title => 'Are you sure?',
                  -text  => "You have requested rm \*\nAre you sure?",
                  -default_button => 'No',
                  -buttons        => ['No','yes']
     my $choice = $D->Show;  # use Show for Tk-b9.01
 # if using Tk-b8:    my $choice = $D->show;
     print " you chose $choice \n";
 A question concerning configuration of the Subwidgets on the Dialogs came
 up recently: 
 <> wrote:
 ! I want to reconfigure the colors of the Dialog and
 ! ErrorDialog buttons.  How do I do this?
 ! Thanks in advance.
    $dialog_widget->configure(-background => 'purple'); 
  Since these two widgets are composites you manage them like any 'ol
  widget. If the default delegate subwidget(s) aren't to your liking you can
  always get to individual component widgets of the composite via the 
  ->Subwidget() method. 
  I see these subwidgets: 
    'message' is the label subwidget with the dialog text, and 'bitmap' is the
    label subwidget showing the dialog bitmap
    'error_dialog' is Dialog subwidget, 'text' is text subwidget
  You can even do things like this: 
  to "get to" the label widget of the dialog component of the error_dialog
  Be sure to also check out the "dialog" demo. 
 16.2. Is there a file selector? 
 Yes, there may be several eventually... 
 One distributed with the perl/Tk code kit itself is called and was
 written by Frederick L. Wagner - (based on an original by Klaus
 Another module called was adapted by Alan Louis Scheinine from
 Wagner's It is available from:
 A module called allows one to type in a new (non-existant)
 filename for "Save as..." type operations. It was posted by Mark Elston on 12
 Oct 1995 to the mailing list and is available from:
 A slightly different behaviour is to be had with Brent B. Powers' that was posted to the mailing list on 12 Jan 1996 and
 available from:
 Harry Bochner chimed in with It is available from:
 In general, if there is a feature that you want missing from one of these, or
 some behaviour that you would like to see modified then by all means cp the
 source code to your area and start hacking ;-) 
 16.3. Is there a color editor? 
 There is. Please see 
 or run the Tk/demos/color_editor demo script for more information. 
 16.4. Is there a round Scale? 
 It is not quite a "round Scale" but Roy Johnson has written "" for
 round dial (or speedometer) -like settable widgets. It is available from:
 As well as from the Contrib/ sub-directory of your perl/Tk build directory. 
 16.5. Is there something equivalent to tkerror? 
 Yes there is. Please see the Tk/ module for further
 16.6. Are there Tables? 
 There are least two: 
 Nick's Table
 Nick Ing-Simmons has distributed his own Table widget package with
 Tk-b9.01 (and higher). It is used through a use TK::Table; and 
 $top->Table(); calls. A rather detailed demo of this widget/geometry
 manager's capabilities can be found in the table_demo script (in your 
 Tk-b9.01/ build directory). There is also pod in the perl5/Tk/
 file. You may also browse the perl Tk::Table man page on the web at
 Guy Decoux's BLT_Table
 Guy Decoux <> has ported the popular BLT_Table
 Tcl/Tk tabular geometry manager to perl/Tk. It was known to work with
 Tk-b8. You may obtain the latest version of it either from
 or from a CPAN site in the authors/id/GUYDX/ directory. You may also
 browse the perl BLT_Table man page on the web at
 17. Programming/development tools. 
 There are a number of tools and methods to help you with your perl/Tk
 scripting and development. It is worthwhile to note here that the -w switch is
 recommended as is the use strict; statement near the top of your
 script/program. If it dies and you still cannot decrypt the error message that
 these generate take a look though man perldiag(1). 
 17.1 Is there a Tcl/Tk to perl/Tk translator? 
 Nick Ing-Simmons has written a (rather lengthy) tcl2perl script. It is
 distributed with the perl/Tk build kit. Please handle carefully! (translation: do
 not expect it to translate arbitrary tcl code accurately nor even into the most
 efficient perl/Tk equivalent. Do go over the converted script with care - and
 do not forget -w and use strict;.) Thanks Nick :-) 
 17.2 Is there something equivalent to wish in perl/Tk? 
 The answer is yes. 
 The idea of wish is that you read from <STDIN> and evaluate each statement.
 The standard way to do this in perl/Tk is to use the tkpsh script that comes in
 your perl/Tk build directory. Another elegant way to get wish like behavior in
 perl/Tk is to use rmt which you can find in perl5/Tk/demos in your perl/Tk
 distribution. When you run rmt you already have set up for you so you
 can start typing things like $mmm = new MainWindow; etc. at the rmt:
 prompt. (This use belies the power of rmt which is derived from Ousterhout's
 Tcl/Tk version of rmt [see section 27.2 of his book]. rmt is capable of
 "inserting Tk code" into simultaneously running Tk applications.) 
 A cruder way to get wish-like behaviour with perl/Tk is to run a "perl shell"
 and type in your usual commands, including use Tk; etc. There is a script
 distributed with perl called perlsh which is written quite simply as: 
      $/ = '';        # set paragraph mode
      $SHlinesep = "\n";
      while ($SHcmd = <>) {
          $/ = $SHlinesep;
          eval $SHcmd; print $@ || "\n";
          $SHlinesep = $/; $/ = ''; 
 You can use this during code development to test out little snippets of code. It
 helps to be an accurate typist and the use strict; is optional here :-) 
 KOBAYASI Hiroaki has a more sophisticated wish like perl/Tk "shell" that
 is called EVA. It is available from:*.tar.gz
 17.3. Is there a debugger specifically for perl/Tk? 
 Not for the latest version - but the -w switch and use strict; are always
 helpful with debugging as they provide informative error messages. 
 You can, of course, run under the standard perl debugger using the -d switch
 like so: 
     perl -d myscript
 But it is recommended that you set you breakpoints carefully since just the
 calls to ManWindow->new require many steps. 
 (Older information): Gurusamy Sarathy <> had built a
 PERL5DB file called Tkperldb (which despite the name is for pTk not
 Tk/perl). One must install an early de-bugger then apply a patch to bring the
 debugger up to date. The early debugger kit was available from:*.tar.gz
 And Gurusamy Sarathy notes that the patch to bring the debugger up to date
 is available at: 
  You need a post 5.001m perl that has support for debugging closures. 
  Or you can simply apply:
  to 5.001m. (5.002beta includes all the fixes in the above patch).
 Note that a perl debugger may be invoked within your script with a line like: 
     $ENV{'PERL5DB'} = 'BEGIN { require Tkperldb }';
 See man perldebug(1) for more help. 
 Keep in mind that you are programming in perl after all. The perl debug line
 mode is available to you through executing the following from your shell: 
     perl -de 0
 Whereupon you must enter all the lines of a script including use Tk;.
 (Fancier file reads & evals are possible - but if you are getting that
 sophisticated why not create your own custom PERL5DB file?) When using 
 perl -dwe 0 beware of the emacs like line editing under this debugger, and
 be forewarned that as soon as you type in the MainLoop; statement perl will
 no longer read from <STDIN>. 
 Ilya Zakharevich <> points out that very recent
 perldb versions will allow for simultaneous X and STDIN reads. He also points
 Note that you may use 
     sub myLoop {
       if (defined &DB::DB) {
         while (1) {             # MainWindow->Count
       } else {
 (and I hope the analogous provision will be in MainLoop in 
  tk-b9 - hi, Nick ;-)
 17.4. Is there a GUI builder in perl/Tk? 
 Work has reputedly (January 1996) started on porting a Tcl/Tk GUI builder
 known as SpecTcl for use with perl/Tk. For the Tcl/Tk SpecTcl kit see:]
 and address questions about SpecTcl to <>. 
 In <//" target="new">> Andreas
 Koschinsky <> announced a perl script for use with
 SpecTcl that has some interesting capabilies: 
  24 Mar 1996 22:45:21 GMT
  ... So i wrote a perl-script that can convert project-file (.ui-files) which
  spectcl writes. The script reads the .ui-file and generates an equivalent
 The URL for ui2perl should be something like:
 Somewhat more removed from SpecTcl there is also SWIG. 
 18. Processes & Inter-Process Communication under Perl/Tk. 
 Inter-Process Communication (IPC) is the subject of spawning and
 controlling other programs or "processes" from within perl (sometimes using
 sockets to do so). The subject is briefly discussed in the perlipc(1) man
 page, and was addressed towards the end of Chapter 6 of The Camel. The
 subject is also discussed in the perl FAQ and at Tom Christiansen's ftp site
 (in the various perlipc* files) at:
 as well as the web site at:
 In addition to the usual perl IPC routines Tk allows (at least) three more
 special functions: fileevent (for handling I/O events), send (for
 inter-widget communication), and after (for time control like a sleep
 expressly for widgets). 
  If a packet hits a pocket on a socket on a port,
  And the bus is interrupted as a very last resort,
  And the address of the memory makes your floppy disk abort,
  Then the socket packet pocket has an error to report!
  -Ken Burchill(?) 
 18.1. How does one get Perl/Tk to act on events that are not coming from X? 
 On 22 Nov 1995 (Yaniv Bargury) wrote: 
  I need to write a GUI monitor, that displays the status and controls a set
  of processes running in the background. The idea is to have the GUI
  application start a few child processes, command the children through
  pipes from the GUI to the children, and display the children status
  coming on pipes from the children to the GUI in real time. 
  The GUI must not be busy waiting, because the CPU resources are
  limited. This excludes using the Tk_DoWhenIdle as explained in the
  The usual way to do this is to for the GUI process to have one select()
  in its main loop. That select() should wait for X events or input from
  the pipes leading from the children. 
  How do you do this? 
 To which Nick Ing-Simmons <> replied: 
  fileevent - it is the hook into the select() in the mainloop. 
 In addition Avi Deitcher <> replied with: 
  I wrote something similar to effectively do a tail -f on multiple hosts,
  displaying the result on separate text widgets. Do the following: 
  with a one-way pipe from each child to the parent. Set up the following: 
  for each pipe that you have. This will cause pTk to monitor the 
  FILEHANDLE and call 'subroutine' when an event happens on that
  handle. In this case: FILEHANDLE = pipename status =
  'readable' or 'writable' or 'exception' and subroutine = any
  subroutine that you want. 
 To provide a concrete example of fileevent usage Stephen O. Lidie wrote a
 wonderful little GUI tail monitor he calls tktail: 
     #!/usr/local/bin/perl -w
     # tktail pathname
     use English;
     use Tk;
     open(H, "tail -f -n 25 $ARGV[0]|") or die "Nope: $OS_ERROR";
     $mw = MainWindow->new;
     $t = $mw->Text(-width => 80, -height => 25, -wrap => 'none');
     $t->pack(-expand => 1);
     $mw->fileevent(H, 'readable', [\&fill_text_widget, $t]);
     sub fill_text_widget {
         my($widget) = @ARG;
         $ARG = <H>;
         $widget->insert('end', $ARG);
     } # end fill_text_widget
 An example of how one might use such a script would be to create and
 monitor a file foo like so: 
     echo Hello from foo! > foo
     tktail foo &
     echo \"A ship then new they built for him/of mithril and of elven glass\" --Bilbo \
      >> foo
 18.2. Is there a send and do I need xauth? 
 There is a Tk::send, but to use it own must write one's own version of 
 Tk::receive. An example of this may be found in the rmt program
 distributed with perl/Tk. Note that as of Tk-b12 (including the released
 version Tk400.200) the script that receives from a Tk::send must run with
 taint chcecking turned on (i.e. with the -T switch thrown) and it must untaint
 all commands received from the other process. 
 The Tk::send <-> Tk::receive process will work under xhost +
 authority. The security this affords comes from the fact that anyone who
 would want to exploit it would have to know how to write a Tk::receive
 custom tailored to your application (in addition to all the other protocol
 Please note that while you may not need xauth authorization it is
 nevertheless always a good idea. 
 18.3. How can I do animations using after? 
 There is a "toggling button" demo script supplied with Tk called after_demo
 that makes effective use of after(). 
 Terry Greenlaw <> of Encompass Technologies
 posted a character cell animator for the really bored. Here it is in a slightly
 modified form that allows string input from the command line (note too the
 recursive call that doesn't sop up system memory): 
 =head1 NAME
 From: "Terry Greenlaw"  Thu Feb 1 12:02:24 EST 1996
 To: ptk@guest.WPI.EDU
 Subj: A code sample for the REALLY bored
 For everyone with a case of Browser envy after using Microsoft's Internet
 Explorer, here's a perl/tk script only slightly more useful than a script
 to do <BLINK>. Don't know why I wrote it. Don't know why you'd run it.
 Maybe if you were writing a ticker tape application. Or had a weird thing
 for Times Square. Anyway....
 Terry Greenlaw (on-site @ Lockheed Martin)      Encompass Technologies          
     #use strict;
     use Tk;
     $message=join(' ',@ARGV,''); 
     if (!$message) {
         $message="THIS IS A VERY LONG SCROLLING MESSAGE...      ";
         $topmssg="This is the top of the screen";
         $botmssg="This is the bottom of the screen";
     else {
     $top = MainWindow->new;
     $l1 = $top->Label(-fg => 'White', -text => $topmssg);
     $l1->pack(-fill => 'both', -expand => 1 );
     $m1 = $top->Label(-fg=>'Red', -bg=>'black',
                       -textvariable => \$message, 
                       -width => 15 
     $m2 = $top->Label(-wrap=>1, 
                       -fg=>'Green', -bg=>'black',
                       -textvariable => \$message2, 
                       -width=>1, -height=>8 
     $l2 = $top->Label(-fg => 'White', -text => $botmssg);
     $l2->pack(-fill => 'both', -expand => 1 );
     after(100, \&scroll_it);
     sub scroll_it {
         $message =~ /(.)(.*)/;
         ($message2 = $message) =~ s/ /  /g;
         after(100, \&scroll_it);
 (Please note that a script like this is now distributed as "TickerTape" in
 your Tk*/Contrib/ directory.) 
 18.4. How do I update widgets while waiting for other processes to complete? 
 The short answer is either 
     $widget -> update;
     $widget -> DoOneEvent;
 Here is a script that makes use of the first of these methods. Note that instead
 of actually doing something useful the "long running process" is simply a call
 to the perl sleep() function for illustrative purposes: 
     #!/usr/bin/perl -w
     use Tk;
     my $m = MainWindow->new();
     my $l = $m -> Listbox();
     $l -> bind('<Double-1>' => sub{sleepy($l)} );
     my @nuts   = qw(Almond Brazil Chestnut Doughnut Elmnut Filbert);
     for (@nuts) { $l -> insert('end',$_); }
     $l -> pack;
     sub sleepy {
         my $widget = shift;
         print "before 1st sleep \n";
         print "after 1st sleep before delete \n";
         $widget -> delete('active');
         $widget -> update;             # try [un]*commenting this
         print "after delete before 2nd sleep \n";
         print "after 2nd sleep \n";
 18.5. How do you fork on System V (HP)? 
 Kraegeloh Martin <> originally asked: 
 ! Subj: signal handling difference on HP vs. SUN
 ! the following code will fork an xterm with vi in it, and it
 ! will refuse to do so while the first xterm is still running.
 ! works fine on my sun.
 ! On HP however, the second time an xterm is started, NO handler
 ! is called when the child dies.
 ! the code:
 ! ===================== 8< ===============================
 ! $SIG{CHLD}=\&w;
 ! sub w{
 !    $pid=wait;
 !    print STDERR "died: $pid\n";
 !    if ( $have == $pid ) { $have = 0; }
 ! }
 To which a part of Nick Ing-Simmons' response was: 
  I suspect HPUX is SysV-ish not BSD or POSIX. So every time a signal
  fires, it removes the handler - you need to reset it in the handler: 
     sub w{
         print STDERR "died: $pid\n";
         if ( $have == $pid ) { $have = 0; }
  Whether you reset it before/after the wait may be very important ... 
 Then Bjarne Steinsbo <> followed up with: 
  That's not the whole story... Another problem is that SIGCLD interrupts
  the read system call on SysV-ish (I like that word! :-) systems. This
  means that you have to test why "" fails, and act accodingly. A program
  that works on both Sun and HP is: 
        $_ = ;
        $! = 0, next if $! =~ /Interrupted/;
        last if $! or !defined $_;
             print STDERR "child still alive\n";
             if(($pid=fork()) != 0){
                print STDERR "forked $pid\n";
             else {
                exec("xterm -e vi") 
     sub w{
        print STDERR "died: $pid\n";
        if ( $have == $pid ) { $have = 0; }

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Last Update March 27 2014 @ 02:12 PM